1. Field of the Invention
The present invention relates generally to a protection circuit and, more particularly, to a circuit that protects other circuits from high voltage transients and prolonged high voltage faults.
2. Description of the Related Art
In electric power distribution systems, circuit breakers are used to protect power lines and other distribution equipment from excessive current flows which tend to overheat and damage current conductors. Circuit breakers carry and monitor the current flowing through the power distribution lines. When the current reaches an undesirably high level in a particular line, a tripping device associated with the circuit breaker interrupts current flow in the power line, usually by energizing an electromagnetic relay connected in series with the line. Each circuit breaker includes current monitoring circuitry, trip determination circuitry, and circuitry to power the tripping device. Moreover, many circuit breakers used in industrial applications offer sophisticated delay circuitry to maximize power dissipation and to prevent unnecessary tripping, alarm or annunciation circuitry, and other types of fault detection circuitry, such as circuitry for ground fault detection.
A power supply is needed to provide adequate power to the circuitry of the circuit breaker for proper operation. Since circuit breakers monitor the power in the distribution lines, power is conveniently drawn from the distribution lines to provide power for the power supply. Circuit breakers of this type are commonly referred to as "fault-powered" circuit breakers. Advantageously, fault-powered circuit breakers operate over a wide range of line current to ensure proper operation of the circuit breaker. For instance, under ground fault conditions, when current is drawn from the line through an external conductor, such as when the conductor comes into contact with a motor casing, the line current is low. However, the power supply must still be able to adequately power its associated circuitry and the tripping relay in order to isolate the affected power line and prevent the ground fault from causing damage. Antithetically, a "short" circuit may produce currents in the power line which are many times greater than the rating of the line. To prevent damage caused by this type of severe overcurrent condition, the power supply contains surge protection so that it continues providing power to the associated circuitry and to the tripping relay in order to isolate the affected power line and prevent damage to the power line and any peripheral devices.
While fault-powered circuit breakers offer many advantages over externally powered circuit breakers, they also present some drawbacks. For instance, some fault-powered circuit breakers cannot provide adequate power particularly during low current situations. At such times the available power is insufficient to energize electromagnetic tripping relays which draw a significant amount of power during actuation. Therefore, an external power supply is coupled to the tripping circuitry. During low current situations, the tripping circuitry draws power from the external supply and, thus, receives adequate power to remain fully functional.
Unfortunately, external power supplies may exhibit spurious high voltage transients, which are typically less than one millisecond in duration. One of these high voltage transients, if received by the electronic tripping circuitry, can produce a malfunction or damage the tripping circuitry. Additionally, external power supplies can experience a high voltage, high energy catastrophic fault. Such a fault delivers a high voltage of long duration to the tripping circuitry. Of course, a prolonged exposure to high voltage will almost surely damage the tripping circuitry.
To complicate matters further, the space allocated to electronic tripping circuitry in circuit breakers makes it difficult to package enhanced functionality accessories in the available space. Thus, a problem arises regarding the placement of protective circuitry, such as fault protection circuitry to solve the problems mentioned above.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.